Variations in daptomycin-beta-lactam synergy and enterococcal species - Daptomycin-ß-lactam synergy depends on enterococcal species and specific mutation

It's not all about the MIC: Daptomycin synergy with ampicillin varies between enterococcal species where cardiolipin synthase mutations confer daptomycin resistanceIt's not all about the MIC: Daptomycin-ß-lactam synergy depends on enterococcal species and specific mutationG. McKew*1,21NSW Health Pathology2 The University of Sydney, Faculty of Medicine and HealthIntroductionIn vitro studies and clinical cases suggest that daptomycin (DAP) in combination with ?-lactams may prevent resistance or achieve synergistic activity in serious enterococcal infections(1)(2-4). However, this has not been studied in vitro with all types of mutations that lead to daptomycin non-susceptibility (DAP-NS) in both major pathogenic enterococcal species. Common DAP-NS-associated mutations occur in the liaFSR and YycFGHIJ cell envelope homeostasis systems(5), and cardiolipin synthase (cls), a membrane-associated phospholipid-processing protein(6). For DAP-NS isolates in vitro, ampicillin restores DAP bactericidal activity, and enhances binding, for E. faecium (Efm) isolates with liaFSR or liaFSR/cls mutations, but not YycFGHIJ /cls(5); studied E. faecalis (Efs) isolates didn't have cls mutations(7). In Efm, cls mutations were associated with higher MICs and in conjunction with liaFSR or YycFGHIJ mutations, and it was suggested that cls mutations were one “hit” in a stepwise pathway. ObjectiveTo explore DAP bactericidal activity and possible synergy with ?-lactams, for high-MIC DAP-NS Efs and Efm cls-only mutants, and an additional DAP-NS isolate without associated mutations. MethodsThree pairs of isolates were studied: one Efs endocarditis isolate with experimentally- induced DAP-NS(8); the others were from persistent VREfm infections before and after DAP, and DAP-AMP therapy. Isolates were sequenced on AB SOLiD 5500 and analysed with CLC Genomics Workbench 9. Time-kill studies (13) were performed with DAP, ampicillin, and DAP-AMP(11); and ceftriaxone and DAP-ceftriaxone for Efs. ResultsIsolateNameDAP ETest MIC µg/mLMLSTMutations a/w DAP-NS EfsO2173NoneA7 48173clsN77_Q79 deletionVREm pair 1T_S278NoneT_NS678NoneVREm pair 2C_S2192NoneC_NS32341clsAla20AspTable 1. DAP MIC, MLST and DAP-NS-associated mutationsDAP-ceftriaxone demonstrated bactericidal activity and synergy for A7. C_NS demonstrated less growth for DAP-AMP than DAP alone, but growth overall. T_NS demonstrated a similar kinetic profile to T_S, with bactericidal activity of DAP, and therefore no discernible DAP-AMP synergy.ConclusionsHigh-level DAP-NS can arise with cls mutations alone, not in a stepwise fashion, and can lead to treatment failure. cls-mediated DAP-NS in Efm can evolve in the presence of ampicillin, commensurate with the finding of absence of synergy in vitro.Efs A7 and VREfm C_NS had the same MIC but markedly different kinetic profiles in the presence of DAP-ßlactam: C_NS grew in DAP-AMP, but A7 demonstrated bactericidal activity for DAP-CRO, and increased killing with DAP-AMP compared to AMP alone. There may be utility of DAP-?lactam combinations for cls-mediated DAP-NS Efs isolates.1. Smith JR, Barber KE, Raut A, Aboutaleb M, Sakoulas G, Rybak MJ. ß-Lactam combinations with daptomycin provide synergy against vancomycin-resistant Enterococcus faecalis and Enterococcus faecium. The Journal of antimicrobial chemotherapy. 2015;70(6):1738-43.2. Desai H, Wong R, Pasha AK. A Novel Way of Treating Multidrug-resistant Enterococci. North American journal of medical sciences. 2016;8(5):229-31.3. Sierra-Hoffman M, Iznaola O, Goodwin M, Mohr J. Combination therapy with ampicillin and daptomycin for treatment of Enterococcus faecalis endocarditis. Antimicrobial agents and chemotherapy. 2012;56(11):6064-.4. Munita JM, Mishra NN, Alvarez D, Tran TT, Diaz L, Panesso D, et al. Failure of high-dose daptomycin for bacteremia caused by daptomycin-susceptible Enterococcus faecium harboring LiaSR substitutions. Clinical infectious diseases : an official publication of the Infectious Diseases Society of America. 2014;59(9):1277-80.5. Diaz L, Tran TT, Munita JM, Miller WR, Rincon S, Carvajal LP, et al. Whole-genome analyses of Enterococcus faecium isolates with diverse daptomycin MICs. Antimicrobial agents and chemotherapy. 2014;58(8):4527-34.6. Davlieva M, Zhang W, Arias CA, Shamoo Y. Biochemical Characterization of Cardiolipin Synthase Mutations Associated with Daptomycin Resistance in Enterococci. Antimicrob Agents Chemother. 2013;57(1):289 - 96.7. Hindler JA, Wong-Beringer A, Charlton CL, Miller SA, Kelesidis T, Carvalho M, et al. In vitro activity of daptomycin in combination with ß-lactams, gentamicin, rifampin, and tigecycline against daptomycin-nonsusceptible enterococci. Antimicrobial agents and chemotherapy. 2015;59(7):4279-88.